This invention relates to a sputtering system for applying to the surface of an objective to be treated a thin film of uniform thickness of a metal or an alloy of discimilar metals with a high degree of productivity.
Generally, sputtering is a process for depositing a thin film of a metal on to a surface in which metallic atoms are sputtered from the cathode by the impact of positive ions when a glow discharge is set up and coated on the surface of an objective located in the vicinity of the anode. A sputtering system utilizing this phenomenon is known as from Japanese Patent Publication No. Sho 56-54392.
A sputtering system of the prior art in which an objective to be treated (hereinafter workpiece) is located outside a target has suffered the disadvantage that difficulties are experienced in achieving a high degree of uniformity in the thickness of a film applied by sputtering because the amount of material deposited on a portion of the surface facing the target is large but the amount of material deposited on a portion of the surface opposite the target is small. This problem might be solved by rotating a workpiece. However, even if the workpiece is rotated, the amount of material deposited is reduced when the surface of the rotating workpiece is not disposed in a position in which it directly faces the target because the metallic atoms fly in one direction. Thus, the deposition rate of film decreases and the forming efficiency of the film by sputtering is reduced.
When it is desired to apply to the surface of a workpiece a thin film of a metal, such as titanium, chromium, silicon, boron, tantalum, zirconium, etc., or a alloy of over two of these metals composed of a nitride, carbide, or oxide thereof, the end can be attained by using a target of complex composition constituted by all the components of the alloy. However, the use of such target of complex composition raises the problem that production cost is high. Moreover, the problems that the system is unable to achieve a uniform thickness in the applied film and low in sputtering efficiency remain unsolved.
A sputtering system is known in which a magnetic field is utilized to trap electrons near the target therein to improve discharging efficiency and produce plasma of high density to accelerate film formation, and in which magnetron discharge is utilized to inhibit a rise in the temperature of a workpiece (substrate). However, this system has suffered the disadvantage that, when the workpiece is formed beforehand with a first hardened material layer by nitriding treatment and then a second hardened material layer (thin film) is formed by sputtering treatment, the surface of the first hardened material layer might be oxidized before the second hardened material layer (thin film) is formed or air or gas might be trapped between the two hardened material layers, thereby preventing the hardened material layers intimately adhering to each other with high bonding strength.
Proposals have been made to avoid this disadvantage by carrying out ion nitriding and sputtering continuously in the same vacuum reaction furnace. In this case, when ion nitriding is performed, a glow discharge is set up by applying a DC voltage between a workpiece serving the cathode and furnace wall or target serving as the anode. Stated differently, reverse sputtering is performed in which the direction in which a DC voltage is impressed is opposite the direction in which a DC voltage is impressed when sputtering is performed. When a DC voltage is applied in a direction opposite the direction in which it is applied in performing sputtering, the treating temperature should be kept at about 500.degree.-570.degree. C. When a rise in temperature is effected merely by relying on glow discharge, the power with which a glow discharge is set up or the value of a discharge current should be increased in value if the workpiece has a large surface area. When the workpiece is complex in shape, or when a large number of workpieces are treated, the glow discharge would become unstable because of a large arc discharges involved. Particularly, when the workpiece is of complex shape, the temperature distribution on the surface thereof might become noneven, and it would be impossible to provide a nitride layer of a uniform thickness.